Process for producing silver halidecasein photographic emulsions



United States Patent 3,144,335 PROCES FOR PRODUCING SKLVERHALIDE- CASEIN PHOTGGRAPHIC EMULSIONS Jerome L. Reid, Cambridge, and Ernst A. Steigmann, Winchester, Mass, assignors to Polaroid Corporation, Cambridge, Mass, a corporation of Delaware No Drawing. Filed Mar. 6, 1961, Ser. No. 93,307 Claims. (Cl. 96-94) This invention relates to photography and, more particularly, to novel photosensitive products and processes for the fabrication of same. It is specifically concerned with the preparation of photographic emulsions having silver halide as the sensitive element. The silver halide is prepared in dispersed form in gelatin. The gelatin is then displaced, subsequent to substantial silver halide crystal formation, by casein, preferably a casein derivative which comprises the reaction product of casesin and an oxidizing peracid. The thus-prepared casein silver halide dispersion possesses the properties of being coagulated by pH adjustment to form granules or agglomerates of silver halide-casein. The silver halide-casein granules are formulated into a photographic emulison by redispersion of the granules in a gelatin or the like matrix or vehicle.

The most common method of preparing photographic silver halide emulsions is by reacting a water-soluble silver salt, such as silver nitrate, with at least one water-soluble halide, such as potassium or sodium bromide, preferably together with potassium or sodium iodide, in an aqueous solution of a gelatin colloid peptizing agent. The dispersion of silver halide thus formed contains watersoluble salts as a by-pproduct of the double decomposition reaction, in addition to an unreacted excess of either of the initial salts. It has been considered desirable in the case of photographic emulsions that the water-soluble salts present in the emulsion be reduced in concentration. it hsa been generally the conventional practice, where gelatin has been used as a protective colloid, to remove the soluble materials therefrom by setting the gel dispersion by means of chilling, noodling the so-set dispersion, and washing the noodles with cold water.

It is a primary object of the present invention to provide novel methods for the fabrication of silver halide photographic emulsions.

A further object of the present invention is to prepare certain specified dispersions of silver halides useful in the prepartion of photographic silver halide emulsions.

A still further object of the present invention is to propare certain specified dispersions of silver halides from which Water-soluble by-products may be readily removed.

A still further object of the present invention is to provide a method of washing specified silver halide dispersions in which substantially all of the liquid may be removed, thus making for a substantially complete removal of any Water-soluble impurities therein.

Other objects of the present invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation and order of one or more of such steps with respect to each of the others, and the product possessing the features, properties and relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

Patented Aug. 11, 1964 ice The term gelatin as utilized herein is intended to signify the art-accepted designation of the photographic colloid binder derived from collagen and comprising a proteinaceous substance, The term is specifically intended to include both acid and alkali process gelatins.

The term casein as utilized herein is intended to signify the art-accepted designation of the white amorphous phosphoprotein occurring in the milk of various animals and the water-soluble metal derivatives thereof, that is, caseinates, for example, alkalimetal derivatives such as sodium caseinate.

Each of the terms gelatin and casein is also intended to include any other product substantially identical therewith, as for example, Where such product is produced synthetically. I

The facts that certain specified derivatives may be prepared from the gelatin employed as a peptizer for preparing dispersions of silver halide and that dispersions of silver halide in those derivatives are acid coagulable and will form silver halide-gelatin derivative granules when the dispersions is adjusted to the proper pH are disclosed in the following US. Patents Nos. 2,481,650, issued September 13, 1949; 2,614,928, issued October 21, 1952; 2,614,929, issued October 21, 1952; 2,728,662, issued December 27, 1955; and 2,956,880, issued October 18, 1960.

The coagulation of silver halide gelatin dispersions by the incorporation of specified additives is disclosed in the following US. Patents Nos. 1,844,716, issued February 9, 1932 (the salt-forming elements of Group IV of the Periodic Table); 2,489,341, issued November 29, 1949 (annion soaps); 2,527,261, issued October 24, 1950 (anion soaps); 2,527,268, issued October 24, 1950 (anion soaps); and 2,618,556, issued November 18, 1952 (ammonium salts, salts of the alkali metals having an atomic weight less than 140, and salts of the metals of Group II of the Periodic Table having an atomic weight less than The coagulation of the aforementioned silver halide gelatin derivative dispersions of US. Patent No. 2,614,- 929, by the addition of the water-soluble salts of cadmium, zinc, aluminum, iron, nickel, chromium, and Ianthanum, is disclosed in U.S. Patent No. 2,768,079 issued October 23, 1956.

These methods avoid the limitations which have previously characterized the preparation of washed silver halide dispersions and are valuable for expanding the possible conditions for the manufacture of photographic silver halide emulsions of different speeds, contrast and other photographic characteristics. In addition, these methods offer simplified means of preparing emulsions of high silver concentration, which are of value in the manufacture of photographic film bearing large quantities of silver per unit area.

The present invention is specifically concerned with the process of fabricating a silver halide photographic emulsion which comprises reacting a water-soluble silver salt, preferably silver nitrate, with at least one watersoluble halide salt, preferably sodium or potassium bromide, and most preferably in combination with sodium or potassium iodide, respectively, in an aqueous solution of a gelatin colloid peptizing agent. Subsequent to substantial silver halide crystal formation, the gelatin colloid peptizing agent is displaced by casein, preferably casein which has been oxidized With an oxidizing peracid, most preferably oxidized with peracetic acid, washed and contacted with alkali to provide a water-soluble alkali metal, for example, sodium or potassium, or ammonium salt thereof. The displacement reaction between the gelatin and the casein derivative employed takes place within a few minutes and the silver halide-casein dispersion possesses the character that it may be coagulated, forming silver halide-casein granules, upon acidification of the mass to a pH within the range of 3 to 5, inclusive, preferably 3.5. The insoluble material which results consists of silver halide grains enveloped by casein and may be washed free of salts and other water-soluble materials which may be present. As these granules readily settle from the liquor solution containing gelatin with which they are in contact, merely dceanting the liquid from the granules removes substantially all the water-soluble salts therefrom. The granules may be subsequently subjected to washing or to redispersing the granules, such as in a solution having a pH at which they will disperse, that is 6 or higher, preferably 6.5, and again coagulating, by lowering the pH, and removing the liquid from the granules thus formed a second time. As previously stated, the coagulation of the silver halide-casein dispersion is effected by acidifying the mass to a pH within the range of 3 to 5, whereby the granules of silver halide and casein will separate out.

The silver halide crystals may be prepared in the gelatin colloid peptizing agent by any of the normal methods for the preparation of silver halide dispersions, such as, for example, by introducing a stream of an aqueous solution of silver nitrate and a stream of an aqueous solution of at least one alkali metal halide, preferably potassium or sodium bromide and potassium or sodium iodide, into a constantly agitated solution of the gelatin peptizing agent. Instead, however, the gelatin solution may be combined with one of the reactants, and the other reactants may be introduced therein with stirring. After the silver halide has been formed in the gelatin colloid peptizing agent, any ripening operations considered advisable may be effected. Casein, preferably casein which has been oxidized with an oxidizing peracid and thus inertized, is then added to the dispersion in order to displace the gelatin colloid peptizing agent and provide the function of rendering the dispersion coagulable at a pH within the range of 3 to 5. The thus-formed silver halide-casein dispersion forms granules upon pH reduction to within the coagulating pH range and quickly settles from the liquid portion of the mass. The coagulum thus formed may then be separated from the liquid by any one of several techniques. In the simplest form, the supernatant mother liquor may be removed from the receptacle in which the mass is contained by means of a siphon or by decantation.

For some types of emulsions, the coagulum may be used directly without further washing. Where it is desired to further reduce the content of soluble salts which may be present, any one of several methods may be employed. For instance, the coagulum may be rinsed by treatment with cool water, preferably below 20 C. and preferably with the pH thereof adjusted to the range of the caseins coagulation pH. In order to provide preferred washing action, the fioc may be suspended in the wash water by agitation. The specific number of rinses which is desirable to reduce the salt content may be determined by experiment, but usually in this method 1 to 4 rinses are sufficient. As an alternative method, the coagulum may be redispersed in water at an elevated temperature, for example, 35 to 60 C., preferably 40 C., using a small amount of an alkaline material, such as sodium hydroxide or ammonium hydroxide, so that the pH of the mixture is of the order of 6 or higher. The temperature of the solution is thus raised to approximately 40 C., and the whole is stirred for a few minutes to effect redispersion of the coagulum. The casein silver halide granules may again be precipitated by the addition of an appropriate acid to reduce the pH to the coagulation point of the derivative. The coagulum will settle and separation from the mother liquor may be effected as previously described. This redispersion and coagulation may be repeated as many times as is necessary. As a third method of washing, the coagulum may be redispersed in an aqueous solution at a pH below 2 so as to effect redispersion of the silver halide-casein granules. The low pH may be obtained by the addition of a quantity of an appropriate acid, such as sulfuric acid. Recoagulation may then be effected by the addition of a suitable base, such as sodium or ammonium hydroxide, to raise the pH of the solution to the coagulating point of the derivative. The coagulum will settle and the separation of the supernatant liquor is carried out as previously described. The coagulum, after the washing operation, consists of silver halide, the casein, any other water-insoluble materials which may be present, and a small amount of residual soluble salts and/ or gelatin dissolved in any water trapped in the coagulum.

The oxidizing peracids employed to provide the preferred oxidized casein are acids which possess a maximum concentration of oxygen, for example, inorganic peracids such as perchloric acid and organic peracids containing a percarboxyl group (COOOH) such as performic and peracetic acids. Peracetic acid, CH COOOH, comprises the preferred acid particularly suitable by reason of its photographic inertness.

In the preparation of the desired oxidized casein, the concentration of the oxidizing peracid employed generally comprises an amount in excess of that concentration stoichiometrically necessary to effectuate inertization of the casein. Preferably, the concentration of the peracid employed is within the approximate range of 0.5 to 2% by weight of dry casein.

The reaction between the casein and the chosen peracid is continued for a sufficient time interval in order to etfect destruction of the most minute photographically active components of the caseins phosphoprotein molecule or impurities therein. Deleterious sulfur-containing compounds such as cystine apparently are preferentially destroyed by the peracid treatment. Thus measurement of the concentration of compounds such as cystine may be employed in general to designate the degree of the reactions completion. Although the time and temperature at which the reaction is maintained may be varied over a considerable range, according to the desires of the operator, the temperature employed will generally fall within the range of 5 to 15 C. and the time of reaction within the range of 12 to 36 hours.

Excess unreacted peracid may be removed or neutralized by treating the casein with sulfurous acid or potassium metabisulfite, generally in an amount comprising approximately 1.5 to 3 parts of sulfurous acid or the salt equivalent per parts of dry casein. By-products of the last-mentioned reaction, that is, sulfuric acid and, when peracetic acid is employed as the oxidizing peracid, acetic acid or the like, may be separated from the casein by neutralizing to pH 4 and then washing the curd according to procedures known in the art.

The photographic inertness of the resultant casein may be readily determined by constituting a silver iodobromide casein emulsion employing, as the protective colloid, the casein; subjecting the thus-formed emulsion to an after-ripening; and measuring the increased sensitivity and/or fog resulting therefrom per unit time. An inert casein will provide substantially no increase in the emulsion sensitivity and/ or fog level.

For the acid which is to be employed for reducing the pH to the coagulation point, any acid may be employed to give the desired pH to the mass which will not deleteriously affect the photographic material. Dilute aqueous sulfuric acid has been found to be particularly useful for this purpose. Nevertheless, other acids, such as phosphoric acid, may be employed.

It is apparent that one advantage of the flocculation" technique for the preparation of photographic emulsions described herein is that the concentration of silver halide in the final emulsion may be adjusted to any desired value without reference to the quantity of water, silver halide and gelatin employed in the initial reaction for the preparation of the silver halide dispersion. Thus, concentrated emulsions may be readily prepared. Dried emulsions may be prepared for storage and shipment without any necessity of extensive evaporation of water or reduction of the water content of the dispersed silver halide and without the necessity of using concentrated salt solutions or organic solvents. The coagulum prepared by the use of casein is especially suitable for the preparation of dried emulsions, since it may be obtained in a state nearly free of excess water, and therefore the drying operation is simplified and rendered less expensive. The coagulum thus may be obtained in a suitable form for this purpose, and the dried coagulum may be readily redispersed in an aqueous composition containing appropriate alkali at any time.

At this point, acknowledgement should be made of the prior art processes which disclose: (1) the employment of casein photosensitive silver halide emulsions such as are disclosed in US. Patents Nos. 705,643 and 797,458, issued July 29, 1902 and August 15, 1905, respectively, wherein the emulsions protective colloid comprises casein; (2) the employment of hydrogen peroxide oxidized casein as the peptizing colloid utilized for the formation of silver halide crystal photographic emulsion dispersions such as is disclosed in U.S. Patent No. 2,691,582, issued October 12, 1954; and (3) the employment of casein, which has been treated with an aqueous solution of an alkali metal pyrophosphate, as the peptizing colloid utilized for the formation of silver halide crystal photographic emulsion dispersions such as is disclosed in US. Patent No. 2,960,405, issued November 15, 1960.

As previously mentioned, in the preparation of photographic dispersions of silver halides in a polymeric colloid binder, the usual procedure has involved the formation of the silver halide crystals in an aqueous solution of gelatin, as the binder. Gelatin has been widely used because it possesses many desirable properties, especially its sensitizing elfect on the silver halide grains and its influence on the crystal growth of the silver halides. This high sensitivity of gelatin-silver halide emulsions is believed to be partially due to the presence in the emulsions of flat, well-shaped, geometric silver halide crystals. The growth of the grains during the conventional ripening procedure employed to formulate the desired dispersions can be followed by microscopic examination. In general, the longer ripening times tend to produce the larger crystals which are especially useful in high-speed photographic emulsion layers.

Prior art attempts to replace the use of gelatin, as the peptizing colloid binder, by casein, for example the products and processes of the previously recited patents, or various synthetic polymeric substances, have singularly failed to achieve extensive commercial utilization in the manufacture of photographic emulsions at present, due at least in part to the retardation elfects of casein and/ or its derivatives on silver halide crystal growth, especially the production of the relatively large silver halide crystals employed in high-speed emulsions.

By employment of the instant processes, the advantageous results, for example the desired silver halide crystal formation, obtained by use of gelatin as the peptizing colloid are maintained and, in addition, a method is provided for replacement of the gelatin peptizing colloid by casein.

The disclosed method, in addition to providing an expeditious method of separating the silver halide dispersion from the undesired water-soluble salt by-products of its formation, provides a silver halide-casein dispersion which is readily compatible with various bulk polymers employed to provide the vehicle or matrix for the preformulated silver halide dispersions.

temperature for the selected time interval.

As is well known, silver halide gelatin dispersions exhibit relatively poor compatibility with the various natural and synthetic polymeric materials, other than gelatin itself, which are considered desirable as bulk protective matrices for the silver halide dispersions, by reason of their gelatin content. However, the silver halide-casein dispersions formulated by the instant processes have been found to be compatible with various of the colloid vehi cles advanced to replace the use of gelatin or its derivatives for this purpose. Examples of such gelatin substitutes include: water-permeable polyamides such as salts of N-carboxyalkoxymethyl polyamides and polyamide ethers; water-permeable cellulose derivatives such as partially hydrolyzed cellulose acetate; water-permeable polymers and interpolymers of acrylic acid and acrylamide; and water-permeable synthetic hydroxyl-containing polymers such as hydrolyzed or partially hydrolyzed vinyl ester polymers and interpolymers, for example, partially hydrolyzed polyvinyl acetate, polyvinyl alcohol and acetals thereof.

It should be further noted, however, that in order not to induce photographically deleterious additives to the emulsions formulated according to the instant processes, the preferred casein employed is substantially completely inertized by reaction with a peracid. Conventional agents employed in the prior art to inertize photographic protein colloids, for example, hydrogen peroxide, sodium peroxide, as well as permanganates, free halogens, and hypochlorites, are not sufficiently powerful to provide the desired inertization. In addition, certain of these agents incorporate undesirable components into the resultant emulsions, for example, the residual halides from free halogens and hypochlorites provide halide concentration emulsion imbalance and permanganates provide silver halide reducing manganese ions.

The invention will be illustrated in greater detail in conjunction with the following specific examples which set out representative preparations of the novel emulsions of this invention, which, however, are not limited to the detailed description therein set forth and are intended to be illustrative only.

A silver halide gelatin dispersion was prepared as follows.A solution comprising 34 grams of potassium bromide, 2.1 grams of potassium iodide, and 4.0 grams of gelatin in 200 cc. of water was mixed with a solution comprising 43 grams of silver nitrate in 400 cc. of water at 60 C. The resultant mixture is then diluted to 1 liter with water at 60 C.

A casein derivative was prepared as f0ll0ws.-200 grams of casein were treated with 2 liters of 1% peracetic acid for 18 hours. The peracetic acid was then filtered off after adjustment of mixtures pH to 5.0i0.5. The casein was then washed with cold water on a Buchner funnel, contacted for 15 minutes with a solution comprising 0.1 g. of potassium pyrosulfite (K S O and then further washed until negative Luminol, S0 and Pbcystine tests were obtained. The casein was then dried overnight.

The dry casein was dissolved in a minimum amount of 1N sodium hydroxide and then adjusted to pH 6.5 with 2.5% acetic acid for use in the following processes.

Flocculazion of a silver halide casein derivative dispersion by displacement of gelatin was provided as follows. To a chosen aliquot of the silver gelatin dispersion was added a predetermined concentration of the casein derivative.

The resultant mixture was stirred at the predetermined The pH of the mixture was lowered to the coagulating range with 10% sulfuric acid and agitated until the casein derivative and silver halide fiocculated, leaving a clear or slightly turbid mother liquor. (Flocculation generally occurs promptly but may require one hour or more for completion. The flocculation rate is enhanced by maintaining the mixture at an elevated temperature, for example, in

the range of 35 to 60 C.) The supernatant liquid was decanted and the floc was washed with distilled Water until the desired pAg or conductivity was attained.

The resultant fioc readily redissolves on warming, adjusting to pH 6 and higher, and diluting to approximately 8 stabilized with gold salts as described in U.S. Patents Nos. 2,597,856 and 2,597,915.

The emulsions may also be chemically sensitized with reducing agents such as stannous chloride as described in U.S. Patent No. 2,487,850; amines such as diethylenetrioriginal volume or higher with bulk gelatin, gelatine 5 amine as described in U.S. Patent No. 2,518,698; polyderivative, casein, or any substitute bulk photographic amides such as spermine as described in U.S. Patent No. protective colloid, to provide the desired photographic 2,521,925; or bis-(B-aminoethyD-sulfide and its wateremulsion. soluble salts as described in U.S. Patent No. 2,521,926.

The following compilation in tabular form sets forth 10 The emulsions may also be stabilized with the mercury specific examples of the last-mentioned processes concompounds of U.S. Patents Nos. 2,728,663; 2,728,664; solidated in the interest of clarity. and 2,728,665.

Volume of the Reaction Example silver Quantities of the temper Reaction Ccagucoagulating Degree of No. halide casein derivative ature, time lating time precipitation gelatin 0. pH disperston, cc.

100 1 cc. of 8% G 151nius Instantaneous. Partial. 100 2 cc. 60 15 mins. -do Substantial. 100 3 cc. G0 15 rnins Almost complete. 100 4 cc. 60 15 mins. Com lete. 100 5 cc. 60 15 mins 13 100 cc. oi10% 60 15 mins Do 100 15 cc. of 8% 60 15 mins Do. 100 15 cc. ol'10% -30 2 hrs Substantial.

The gelatin displaced by the casein derivative and remaining in solution in the clear supernate can readily be recovered by precipitation techniques well known in the art. For example, solid ammonium sulfate may be added to the supernate to effect precipitation of the gelatin. The sulfate may then be extracted from the gelatin precipitate which, in turn, readily redissolves upon heating.

The resultant emulsions may be sensitized, the desired additives incorporated therein, and the emulsion coated and processed according to conventional procedures known in the emulsion manufacturing art.

The emulsions of this invention may be coated onto various types of rigid or flexible supports, for example, glass, paper, metal, polymeric films of both the synthetic types and those derived from naturally occurring products, etc. Especially suitable materials include paper; aluminum; polymethacrylic acid methyl and ethyl esters; vinyl chloride polymers; polyvinyl acetals; polyamides such as nylon; polyesters such as the polymeric films derived from ethylene glycolterephthalic acid; and cellulose derivatives such as cellulose acetate, triacetate, nitrate, propionate, butyrate, acetate-propionate, or acetatebutyrate.

The light-sensitive material of the photographic emulsions comprises a compound of silver, for example, one or more of the silver halides of which silver chloride, silver bromide and silver iodide are examples. The preferred silver halide emulsion comprises a silver iodobromide emulsion. It will be understood that preferred silver halide emulsions of varying halide concentration may be advantageously employed.

The emulsions of the present invention may be chemically sensitized by any of the accepted procedures. For example, the emulsions may be digested with naturally active gelatin, or sulfur compounds can be added such as those described in U.S. Patents Nos. 1,574,944; 1,623,499; and 2,410,689.

The emulsions may also be treated with salts of the noble metals such as ruthenium, rhodium, palladium, iridium, and platinum, all of which belong to Group VIII of the Periodic Table of Elements and have an atomic weight greater than 100. The salts may be used for sensitizing in amounts below that which produce any substantial fog inhibition, as described in U.S. Patent No. 2,448,- 060 and as antifoggants in higher amounts, as described in U.S. Patents Nos. 2,566,245 and 2,566,263.

The emulsions may also be chemically sensitized with gold salts as described in U.S. Patent No. 2,399,083 or The emulsions may also be optically sensitized with cyanine and merocyanine dyes as described in U.S. Patents Nos. 1,846,301; 1,846,302; 1,942,854; 1,990,507; 2,112,- 2,165,338; 2,493,747; 2,493,748; 2,503,776; 2,519,- 001; 2,666,761; 2,734,900; 2,739,149; and 2,739,964.

The emulsions may also contain speed-increasing compounds of the quaternary ammonium type as described in U.S. Patents Nos. 2,271,623; 2,288,226; and 2,334,864; and of the polyethylene glycol type as described in U.S. Patent No. 2,708,162.

Where desired, suitable antifoggants, restrainers, accelerators, preservatives, coating aids, and/ or stabilizers may be included in the composition of the emulsions.

Hardening agents such as inorganic agents providing polyvalent metallic atoms, especially polyvalent aluminum or chromium ions, for example, potash alum [K Al (S0 .24H O] and chrome alum [K Cr (SO .24H O] and organic agents of the aldehyde type such as formaldehyde, glyoxal, mucochloric acid, etc., the ketone type such as diacetal, and the quinone type, may be incorporated in the emulsions according to procedures well known in the art.

The term photosensitive and other terms of similar import are herein employed in the generic sense to describe materials possessing physical and chemical properties which enable them to form usable images when exposed to actinic radiation.

Since certain changes may be made in the above process and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of preparing a silver halide dispersion which comprises the steps of mixing together a water-soluble silver salt and at least one water-soluble halide salt in an aqueous solution of gelatin, thereby forming a silver halide-gelatin dispersion, adding casein to said dispersion, thereby displacing said gelatin from contact with said silver halide to form a silver halide-casein dispersion coagulable at a pH within the range of 3 to 5, inclusive, coagulating the thus-formed silver halide-casein dispersion by adjusting same to a coagulating pH, thereby forming silver halide-casein granules, and separating the thus-formed silver-halide granules from the liquid portion of the mass.

2. A method as defined in claim 1, wherein said watersoluble silver salt comprises silver nitrate.

3. A method as defined n claim 1, wherein water-soluble halide salts comprise alkali metal halide salts.

4. A method as defined in claim 1, wherein said casein comprises oxidized casein prepared by oxidizing casein with a peracid.

5. A method as defined in claim 4, wherein said oxidized casein is selected from the group consisting of watersoluble alkali metal and ammonium salts of the reaction product of casein and a peracid.

6. A method as defined in claim 4, wherein said peracid comprises peracetic acid.

7. A method as defined in claim 1, including the step of mixing the separated silver halide-casein granules in a polymeric protective colloid for said silver halide-casein dispersion.

8. A method as defined in claim 7, including the step of washing said separated silver halide-casein granules prior to mixing thereof with said polymeric protective colloid.

9. A method as defined in claim 7, wherein said separated silver halide-casein granules are mixed in said 10 polymeric protective colloid at a pH of not less than 6.

10. A method as defined in claim 7, wherein said separated silver halide-casein granules are mixed in said polymeric protective colloid at a temperature within the range of about 35 to 60 C.

References Cited in the file of this patent UNITED STATES PATENTS 750,048 Brooks Jan. 19, 1904 2,618,556 Hewitson et al Nov. 18, 1952 2,691,582 Lowe et al Oct. 12, 1954 2,890,215 Corben et a1 June 9, 1959 2,960,405 Dersch et a1 Nov. 15, 1960 3,011,890 Gates et a1 Dec. 5, 1961 FOREIGN PATENTS 495,889 Canada Sept. 8, 1953 1,090,082 Germany Sept. 29, 1960 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3, 144,335 August 11, 1964 Jerome L Reid et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 33, for "by-pproduct" read Icy-product line 38, for "hsa" read has column 3, line 13, for "'liquor" read liquid line 14, for "decanting" read decanting column 9, line 1, for "n" read in column 10, line 19, for "1,090,082" read 1,090,092

Signed and sealed this 13th day of April 1965,,

(SEAL) Altest:

ERNEST W. SWIDER' EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A METHOD OF PREPARING A SILVER HALIDE DISPERISON WHICH COMPRISES THE STEPS OF MIXING TOGETHER A WATER-SOLUBLE SILVER SALT AND AT LEAST ONE WATER-SOLUBLE HALIDE SALT IN AN AQUEOUS SOLUTION OF GELATIN, THEREBY FORMING A SILVER HALIDE-GELATIN DISPERSION, ADDING CASEIN TO SAID DISPERSION, THEREBY DISPLACING SAID GELATIN FROM CONTACT WITH SAID SILVER HALIDE TO FORM A SILVER HALIDE-CASEIN DISPERSION COAGULABLE AT A PH WITHIN THE RANGE OF 3 TO 5, INCLUSIVE, COAGULATING THE THUS-FORMED SILVER HALIDE-CASEIN DISPERSIN BY ADJUSTING SAME TO A COAGULATING PH, THEREBY FORMING SILVER HALIDE-CASEIN GRANULES, AND SEPARATING THE THUS-FORMED SILVER-HALIDE GRANULES FROM THE LIQUID PORTION OF THE MASS. 